CMOS low battery voltage detector

ABSTRACT

A system and method for detecting a low battery voltage supplied to a battery operated integrated circuit. A stable reference voltage provided by a bangap reference is compared with the battery voltage. A switched capacitor circuit is used instead of the more conventional resistor combination to supply a scaled representation of the battery voltage. Power requirements are reduced by combining the bandgap reference and the comparator into a single component.

FIELD OF THE INVENTION

This invention relates to battery operated circuits and, moreparticularly, to a method and system for detecting low battery voltageutilizing a stable reference voltage and a switched-capacitor circuit.

BACKGROUND OF THE INVENTION

In many battery operated applications, some means for detecting a lowbattery voltage is needed. In such applications, a warning signal mightbe generated or, alternatively, vulnerable circuits within a systemmight be selectively disabled by an appropriate control signal in orderto avoid damage to or malfunction of such circuits. A straightforwardmethod of implementing a low voltage detection function is to compare ascaled battery voltage and a stable reference voltage such as a voltagefrom a bandgap circuit. It is known, as will be discussed in greaterdetail later, that a bandgap circuit can generate a stable,substantially temperature independent reference voltage. Normaltime-continuous bandgap references typically need well-matched resistorsand transistors and some sort of trimming to get good accuracy.

U.S. Pat. No. 5,196,833, which issued Mar. 23, 1993, to Kemp describes alow voltage detection circuit which includes a bandgap reference and adifferential comparator circuit for generating a signal proportional tothe level of the supply voltage. Since many battery operated circuitsare subject to ambient temperature variations, such as an automobileapplication as disclosed in the U.S. Pat. No. 5,196,833, it is importantthat some form of temperature compensation be used in deriving thereference voltage. The bandgap cell, as disclosed in the aforementionedpatent, includes bipolar transistors in which the base to emittervoltage is used as a reference source. Typically, two transistors areused having different current densities through each. In the U.S. Pat.No. 5,196,833, the different current densities are achieved by usingfour parallel transistors in one current path and a single transistor inthe second. It has been shown that the base to emitter voltage (V_(BE))of a bipolar transistor exhibits a negative temperature coefficient withrespect to temperature. On the other hand, it has also been shown thatthe difference of base to emitter voltages ΔV_(BE) of the two bipolartransistors operating at different current densities exhibit a positivetemperature coefficient with respect to temperature. Thus, the sum ofthe base to emitter voltage V_(BE) of a bipolar transistor and adifferential voltage ΔV_(BE) will be relatively independent oftemperature when the sum of the voltages equals the energy gap ofsilicon. Such temperature stable references have been created bygenerating a V_(BE) and summing a ΔV_(BE) of such value that the sumsubstantially equals the bandgap voltage of 1.205V.

U.S. Pat. No. 5,814,995, which issued Sep. 29, 1998, to Tasdighi,discloses a voltage detector for a battery operated device employing twobipolar transistors and an operational amplifier as a comparator. Thesystem of this reference also employs resistors which, as notedhereinbefore, must be well-matched and usually require some sort oftrimming to obtain good accuracy.

U.S. Pat. No. 4,375,595, which issued Mar. 1, 1983, to Ulmer et al.,relates to a temperature independent bandgap reference which employsswitched capacitors to input the V_(BE) and ΔV_(BE) of the bipolartransistors. A proper selection of the ratio of the switched capacitorsis used to get around the need for matched and/or trimmed resistors. Theswitched capacitor implementation employs clock signals in order toestablish a precharge phase and an output reference stage. In the U.S.Pat. No. 4,375,959, three separate clock signals are required.

U.S. Pat. No. 5,563,504, which issued Oct. 8, 1996, to Gilbert et al.,also relates to a switching bandgap voltage reference in which onebipolar transistor is used with two different current sources providingthe current densities needed to obtain the ΔV_(BE) value.

Thus, in an application where a battery voltage detector is required inorder to detect a low voltage value, and providing that a suitable clockexists within the application, a switched capacitor architecture canprovide an effective solution.

SUMMARY OF THE INVENTION

The present invention provides lower untrimmed errors by utilizingoffset cancellation techniques. This, in combination with a switchedcapacitor network having well-matched capacitors provide suitableweighting factors.

The invention also provides for a reduced component count by combining avoltage reference circuit and a voltage comparator into a singlecircuit.

In a preferred embodiment, the comparator implementation eliminates theneed to actually generate a 1.2V bandgap reference voltage.

The invention also provides for lower power requirements by eliminatingpart of the static current by replacing resistors in the bandgap circuitwith a switched capacitor circuit.

Therefore, in accordance with a first aspect of the present inventionthere is provided a detector for providing a low battery voltageindication comprising: a stable voltage reference and comparator circuitfor comparing the battery voltage and the voltage reference; switchmeans to switch between a first operational state and a secondoperational state; a switched capacitor circuit to store charges relatedto the battery voltage and the reference voltage in each operationalstate; and a clock to initiate switching between the first and secondstates, wherein an output from the comparator during the second stateindicates whether the battery voltage is below a preset threshold.

In accordance with a second aspect of the present invention there isprovided a method of detecting a low battery voltage supplied to anintegrated circuit comprising: providing a stable voltage reference andcomparator circuit for comparing battery voltage against the referencevoltage; providing a capacitor circuit for storing charges associatedwith the battery voltage and the reference voltage; providing switchingmeans to switch between a first phase wherein the capacitors are chargedto a first voltage level, and a second phase wherein the capacitors arecharged to a second level; and comparing the first and second levels todetermine whether the battery voltage is above or below a presetthreshold.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail with reference tothe attached drawings wherein:

FIG. 1 illustrates a prior art voltage detector utilizing resistors;

FIG. 2 illustrates a switched capacitor, temperature-independent,bandgap reference utilizing three input clocks;

FIG. 3 is a simplified schematic of the low battery voltage detectoraccording to the present invention;

FIG. 4 illustrates a second embodiment of the bandgap reference circuit;and

FIG. 5 is a circuit diagram of the CMOS low voltage battery detectoraccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a basic, low battery voltage detector 2 according to theprior art. The scaled battery voltage taken across the voltage dividercircuit comprising resistors 4 and 5 is supplied to the positive inputof an operational amplifier 6 and the voltage reference 7 which in apreferred embodiment is a bandgap reference is supplied to the negativeinput of the operational amplifier 8. The output of the operationalamplifier is 0 if the battery voltage is low and is a 1 otherwise.

A discussed previously, the voltage divider circuit comprising theresistors 4,5 increases the power requirement to the detector circuitand can lead to measurement inaccuracies.

To overcome the inaccuracies created by the resistor combination, aswitched capacitor circuit 10 as shown in FIG. 2 has also been disclosedin the prior art. In this circuit bipolar transistors 12 and 14 are usedto obtain the stable bandgap reference voltage. Comparator 42 evaluatesthe stored voltage across switched capacitors 28 and 34. Clock 16provides clock signals A, B and C to switches 30, 32, 36, 38 and 48.

The present invention provides a low voltage monitoring circuit 100utilizing the switched capacitor arrangement which is illustrated at ahigh level in FIG. 3. The circuit 100 includes switches 110, 112, 114,116, 118 and 120, operational amplifier 122 bipolar transistors 124 and126, current source 128, capacitors 130 (C1), 132 (C2), 134 (C3) andbattery voltage input 136.

As mentioned previously, the object of the invention is to compare anappropriately scaled battery voltage to a stable reference voltage. Thereference voltage from the bandgap reference is the sum of a V_(BE) witha negative temperature coefficient and a K₁ΔV_(BE) with a positivetemperature coefficient, were K₁ is a scaling factor chosen to balancethe negative and positive coefficient and ΔV_(BE) is the difference inV_(BE) between the two transistors with different current densities. Thesum of V_(BE) and K₁ΔV_(BE) is equal to the silicon bandgap voltage orapproximately 1.2 volts.

In essence, the object of the invention is carried out by determiningthe sign of the expression V_(BE)+K₁ΔV_(BE)−K₂V_(BAT). In evaluatingthis expression it is possible to see whether V_(BAT) is above or belowa minimum voltage V min. The value of the Vmin depends on K₂ and isapproximately 1.2/K₂ volts

In a switched capacitor circuit, the above mentioned three voltages canbe sampled on capacitors C1, C2 and C3 and then summed to get theanswer. The size of each capacitor controls the scaling of each voltageand the accuracy of the scaling is normally a magnitude better then ifresistors are used. Another inherent advantage of the present inventionis that the typical 1.2 volt bandgap reference voltage is actually nevercreated, so in principle the circuit will work for supply voltages below1.2 volts. Further, there is not a problem in setting Vmin to be lessthan 1.2 volts.

As discussed previously, a clock signal (not shown in FIG. 3) isrequired in the implementation of the detector circuit. The clock isrequired to generate two phases or stages which are shown in FIG. 3 asphase 1 (Φ1) and phase 2 (Φ2). During phase 1 or the pre-charge phaseswitches 110, 112, 114 and 116 are closed while switches 118 and 120 areopen. In phase 2 switches 110, 112, 114 and 116 are open while switches118 and 120 are closed. All of the above switches are shown in FIG. 1 asgeneric switches for simplicity while it is known that these switches ina preferred embodiment are actually MOS devices as shown in greaterdetail in FIG. 5.

In pre-charge phase 1 switch 114 is closed so that current from currentsource 128 flows through both transistors 124 and 126. Hence bothtransistors are active and the positive input of operational amplifier122 will be biased at V_(BE) also known as V_(BElow). Since switch 116is also closed the negative input of operational amplifier will also beraised to V_(BElow). Since switch 110 and 112 are closed and switch 120open, capacitor 130 and capacitor 132 will be charged toV_(BElow)+V_(offset) where V_(offset) is the off set voltage in theoperational amplifier 122. Capacitor 134 will be charged to V_(offset).

During the second stage or phase 2, switches 110, 112, 114 and 116 areopened while switches 118 and 120 are closed. Now all of the currentfrom source 128 flows through the transistor 126 and this higher currentdensity results in a higher base to emitter voltage across transistor126. The positive input of operational amplifier 122 is now raised toV_(BE2) or V_(BEhigh). As discussed previously the difference betweenV_(BEhigh) and V_(BElow) is the ΔV_(BE) value. It has a value kT/qln(N+1) when N is the relative emitter area of transistor 124.

When the battery voltage V_(BAT) at input 136 is at the threshold ortrip point, V_(min) the negative input of operational amplifier 122 willbe at a voltage equal to V_(BEhigh) plus V_(offset). Capacitor 130 ischarged to V_(BEhigh)+V_(offset)−V_(BAT) while capacitor 132 andcapacitor 134 are charged to V_(BEhigh)+V_(offset). The transfer ofcharges between the capacitors when switching between phase 1 and phase2 can be written has: C3V_(BEhigh)+(C1+C2)ΔV_(BE)−C1V_(BAT). It will berecognized that this is similar to the expression previously given asV_(BE)+K₁ΔV_(BE)−K₂V_(BAT). In this expression the weighting factor K₁will be determined by (C1+C2)/C3 and the factor K₂ is given by C1/C3. Itis significant that the off set voltage in the operational amplifier 122will not affect the above result as long as it does not change when thereference voltage at the positive input changes from V_(BElow) toV_(BEhigh) so a small and simple amplifier can be used to save currentand area.

At relatively low operating voltages MOS switches can have a sufficienton resistance especially if they are biased at a voltage between thesupply voltages. This is not a problem in a switched capacitor circuitas long as the time constants are much shorter then the clock. But theswitch in series with the transistor 124 will have a DC current and thiswill result in a voltage drop and consequently reduced accuracy. Adifferent topography may therefor be used to switch transistor 124 onand off and this alternate topography is shown in FIG. 4. In thisimplementation, two current sources are provided and switch 114 isreplaced with switches 142 and 144. In phase 1 switch 144 is closed andthe switch 142 is open and in phase 2 switch 142 is closed while switch144 is open. Therefore in phase 1 the base of transistor 124 is shortedto ground and the transistor is active. In phase 2 the base oftransistor 124 is biased at 1V_(BE) above ground. If the transistors 126and 140 are matched the value V_(BE) is the same voltage as the emittervoltage at transistor 124 so that V_(BE) is 0 volts and the transistorwill be turned off. There is still current flowing through switch 144 toground during phase 1 but only the base current. With the switchconnected to ground the on resistance will also be lower.

FIG. 5 is a detailed circuit diagram of the CMOS low voltage detectorcircuit including the circuit topography of FIG. 4.

Although specific embodiments of the invention have been described andillustrated it will be apparent to one skilled in the art that numerousvariations can be made without departing from the basic concept. It isto be understood, however that such variations will fall within the truescope of the invention as defined in the appended claims.

What is claimed is:
 1. A detector for providing a low battery voltageindication comprising: a stable voltage reference and comparator circuitfor comparing the battery voltage and the voltage reference; a switchmeans to switch between a first operational state and a secondoperational state; a switched capacitor circuit to store charges relatedto the battery voltage and the reference voltage in each operationalstate; and a clock to initiate switching between the first and secondstates; wherein an output from the comparator during the second stateindicates whether the battery voltage is below a preset threshold (Vmin)such that Vmin=Vref−K*Vbat where: Vref is the reference voltage Vbat isthe battery voltage; and K is a scaling factor.
 2. A detector as definedin claim 1 wherein said stable voltage reference is a bandgap referencecircuit.
 3. A detector as defined in claim 2 wherein said bandgapreference circuit comprises a pair of bipolar transistors supplied witha first current density in said first state and a second greater currentdensity in said second state.
 4. A detector as defined in claim 1wherein said first operational state is a precharge phase and saidsecond operational state is an evaluation phase.
 5. A detector asdefined in claim 1 fabricated on an integrated circuit (IC) employingCMOS technology.
 6. A detector as defined in claim 5 wherein saidcomparator is an operational amplifier.
 7. A detector as defined inclaim 6 wherein offset voltages due to first and second inputs to saidoperational amplifier are cancelled by said switched capacitor circuit.8. A detector as defined in claim 1 wherein capacitors in said switchedcapacitor circuit define a weighting factor for establishing saidthreshold.
 9. A detector as defined in claim 1 wherein said bandgapreference circuit comprises three bipolar transistors with a firstswitchable current source is supplied to one transistor and a secondcurrent source is supplied to the other two transistors.
 10. A detectorfor providing a low battery voltage indication comprising: a stablevoltage reference and comparator circuit for comparing the batteryvoltage and the voltage reference; switch means to switch between afirst operational state and a second operational state; a switchedcapacitor circuit to store charges related to the battery voltage andthe reference voltage in each operational state; and a clock to initiateswitching between the first and second states; wherein an output fromthe comparator during the second state indicates whether the batteryvoltage is below a preset threshold (Vmin) set such that:Vmin>VBE+K1ΔVBE−B2BAT where: Vmin is the threshold value VBE is the baseto emitter voltage across a first transistor of the bandgap reference;ΔVBE is the difference between base to emitter voltages in two bandgaptransistors having difference current densities; VBAT is the batteryvoltage; K1 is a first scaling factor, and K2 is a second scalingfactor.
 11. A detector as defined in claim 10 having three capacitors(C₁, C₂ and C₃) wherein K₁ and K₂ are determined by said capacitorsaccording to the ratios K₁ =C₁ +C₂/C₃ and K₂=C₁/C_(3.)
 12. A method ofdetecting low battery voltage supplied to an integrated circuitcomprising: providing a stable voltage reference and comparator circuitfor comparing battery voltage against the reference voltage; providing acapacitor circuit for storage charges associated with the batteryvoltage and the reference voltage; providing switching means to switchbetween a first phase wherein said capacitors are charged to a firstvoltage level and a second phase wherein said capacitors are charged toa second level; and comparing said first and second levels to determinewhether said battery voltage is above or below a preset threshold (Vmin)such that Vmin=Vref−K*Vbat where: Vref is the reference voltage Vbat isthe battery voltage; and K is a scaling factor.
 13. The method asdefined in claim 12 wherein said switching means are MOS devices. 14.The method of claim 12 wherein said stable voltage reference is abandgap cell having a value of approximately 1.2 volts and Vmin isapproximately 1.2/K .